The invention relates to systems for testing fretting wear and other dissipative mechanical phenomena such as viscosity measurement.
In fretting wear testing, knowledge of the amount of power, i.e. energy per unit time, consumed by frictional losses during the course of a wear test can provide insight into the dynamic processes that occur in fretting wear. This information is particularly useful in studies of surface-modified materials because the physical properties of such materials generally varies with the progression of wear. In viscosity measurement, current methods for measuring fluid viscosity are not suited for use in sensors. Advances in micro-machining techniques allow tiny mechanical structures to be manufactured using photolithographic techniques that are compatible with the manufacture of integrated electronic circuits. A piezo-electrically driven micro-actuator could be integrated with loop control, signal conditioning and temperature sensing circuitry, and an economical, self-contained viscometer sensor may be useful for process control and monitoring in the petroleum, biotechnology and food service industries.
Microscopic damage caused by fretting wear is of significant concern in many engineering applications. The present invention provides a versatile fretting wear testing apparatus and method which has been successfully applied to materials treated by plasma surface modifications, including nitrogen implantation, thin film alloy deposition, and diamond-like carbon film deposition.
Fretting refers to small amplitude, high frequency, oscillatory slip motion between two solid surfaces in contact, "Materials Evaluation Under Fretting Conditions" ASTM STP 780, June 1981, edited by S. R. Brown, Warminster, Pa 1981. The amplitude of the oscillations can vary from fractions of a micron to hundreds of microns, while typical frequencies range from tens to hundreds of Hertz. Fretting wear damage occurs in a number of applications, such as aircraft, automobiles, nuclear reactors, electrical contacts and surgical implants. Although fretting wear damage occurs in localized regions, it is often a precursor to catastrophic failure. Therefore, accurate assessment of fretting wear resistance of materials is of considerable technological and scientific significance.
The small slip amplitudes and high oscillating frequencies associated with fretting wear distinguish it from unidirectional wear. Phenomenologically, this leads to the accumulation of wear debris, products of chemical reaction, and limited heat transfer in the vicinity of the damaged region, G. Dearnaly in "Surface Engineering", edited by R. Kassowsky and S. Singhal (S'-Gravenhague, the Netherlands, Martinus Nyhoff, 1984) p. 125. Studies of the effect of slip amplitude on fretting wear have shown that surface damage can occur at amplitudes well under a micron, "Materials Evaluation Under Fretting Conditions", P. Kennedy, M. B. Peterson and L. Stallings, ASTM STP 780 June 1981, edited by S. R. Brown, Warminster, Pa. The identification of the upper limit of slip amplitude where the transition from fretting to unidirectional wear occurs is a subject of continued research. Systematic investigations of fretting wear damage up to slip amplitudes of 1,000 microns have shown that a dramatic increase in wear rate occurs in the range of 70 microns, suggesting that this might mark the upper limit for fretting wear: "Fretting Corrosion", R. B. Waterhouse, Pergamon Press, N.Y., 1972; N. Ohmae and T. Tsukizoe, Wear, 281, 1974. Other studies have noted a change in morphology and color of wear debris at about 100 microns, suggesting a change in wear mechanism from fretting to one that would be observed in a purely unidirectional wear situation: "Materials Evaluation Under Fretting Conditions", D. Kusner, C. Poon and D. W. Hoeppner, ASTM STP 780 June 1981, edited by S. R. Brown, Warminster, Pa; N. P. Suh, Wear, 44, 1, 1977; J. S. Halliday and W. Hirst, Proc. Roy. Soc. London, A-236, 411, 1956. Even with this systematic approach, a fundamental problem is that fretting wear depends on test conditions, environmental conditions, and the properties of the two contacting materials, H. Czichos and J. Molgaard, Wear, 44, 247, 1977.
The dependence of fretting wear on the oscillating frequency is strongly influenced by environmental conditions. With some materials, it has been noted that fretting wear rates decrease with increasing frequency up to about 30 Hz, H.H. Uhlig, J. App. Mech, 21, 401, 1954. This has been attributed to the relationship between the time for one fretting cycle and the kinetics of oxide film re-growth to a critical thickness where it can be wiped away and act as abrasive debris. The pivotal role of oxidation has been confirmed by high temperature studies where good agreement was reached between fretting wear rates and transition in temperature-dependent oxide growth mechanisms, P. L. Hurricks, Wear, 30, 189, 1974. Furthermore, it has been noted that fretting wear damage was independent of oscillating frequency when the testing was carried out in an inert atmosphere such as dry nitrogen, I. M. Feng and H.H. Uhlig, J. App. Mech, 21, 395, 1954.
The mechanism of material removal in a fretting wear process encompasses the effects of adhesion, abrasion and oxidation, J. E. Elder, R. Thamburaj and P. C. Patnaik, Intl. Materials Rev. 33, 289, 1988. It is believed that the wear process is initiated by localized adhesion of wear debris at surface asperities. The debris accumulates in the wear scar and is work hardened during subsequent cycles. Simultaneous oxidation of the freshly worn surface occurs, leading to the production of oxide particles. The work hardened debris and oxide particles lead to abrasive wear.
Previous designs for fretting wear testers have used a variety of mechanisms for generating the slip motion between the contacting surfaces: "Materials Evaluation Under Fretting Conditions", ASTM STP 780, June 1981, edited by S. R. Brown, Warminster, Pa 1981; J. Gensheimer, G. Friedrich, Wear, 17, 407, 1971. Most actuator designs fall into one of two general categories, mechanical or electromagnetic. Mechanical actuators rely on a rotating shaft in conjunction with a cam or out-of-balance weights to provide a reciprocal motion. The displacement amplitude is usually adjusted mechanically, with the frequency set by the rpm of the rotating shaft. Electromagnetic actuators can be controlled by electronic means and typically exhibit a broader frequency range than their mechanical counterparts. The present invention uses an electromagnetic actuator.
Most tribological studies depend on the observation of the effects of a controlled physical damage to the sample of interest. A purpose of the present fretting wear tester is to simulate the effects of small amplitude oscillatory wear. In one aspect, the present invention simulates the physical conditions that occur in real world situations where fretting wear is a relevant wear mechanism.
Analysis of a fretting wear scar can be accomplished using many of the same techniques common to other tribological tests, including optical and electron microscopy, profilometry and mass loss analysis. The present invention also provides for the in situ measurement of relative friction during the course of a wear process.
Fretting wear phenomena are difficult to study because of the many parameters that can play a significant role in a particular fretting wear process. Aside from the properties of the interacting materials themselves, the parameters that have been identified as being significant in fretting wear include: loading force; lubrication and/or corrosion at point of contact; amplitude of displacement; frequency of oscillation; and number of cycles. The present invention was designed to provide for the control of all of the above parameters in a repeatable manner.
In one aspect of the invention, an electro-mechanical actuator provides small amplitude, oscillatory slip motion between a stylus and a workpiece or fluid in a way that will result in dissipative interaction between the stylus and the workpiece material or fluid to be analyzed. A displacement transducer is used to monitor the amplitude of the stylus displacement providing the frictional oscillatory slip motion. A control loop 60 adjusts the actuator's excitation signal to keep the stylus displacement constant, i.e. constant amplitude slip motion. The amount of excitation power needed by the actuator to maintain a particular displacement, i.e. a constant amplitude slip motion, is related to the amount of dissipative loading caused by friction or viscosity and is described by the equation: EQU W.sub.A =W.sub.O +W.sub.f
where W.sub.A is the power applied to the actuator, W.sub.O is the nominal power required to maintain the displacement without dissipative loading, and W.sub.f is the power consumed by the dissipative processes (friction or viscosity). If the actuator is driven at its resonant frequency, the system becomes particularly sensitive to dissipative loading because W.sub.O becomes very small. The power dissipated by frictional processes is proportional to the mechanical resistance to motion (friction or viscosity) according to the equation ##EQU1## where R is a coefficient of friction, .omega. is the frequency of oscillatory slip motion, and A is the amplitude of the oscillator slip motion or displacement.
In another aspect of the invention, the amplitude and frequency of the slip motion can be independently varied to simulate a wide range of fretting conditions. The power needed by the electromagnetic actuator to maintain a constant displacement amplitude is sensitive to mechanical loading conditions. The amount of power supplied to the actuator is particularly sensitive to mechanical loading when the actuator is driven at its natural resonance, i.e. resonant frequency, as noted above. By providing a means to monitor the actuator excitation signal, a plot of the power expended during fretting wear by frictional processes is obtained.